In the comminution of abrasive material, for example in the manufacture of cement and lime, it is generally known to comminute the material to be processed in a roller mill, the comminuted material emerging over the outer peripheral edge of the grinding path then being raised predominantly with the aid of an upwardly rising air flow into an air separator which is accommodated with the roller mill in a common housing and thus joined to said roller mill to form a unit.
It is also already known (U.S. Pat. No. 3,951,347), for processing material to be ground which contains particles which are difficult to grind, to conduct the fine portion of the material comminuted in the roller mill in the aforementioned manner by means of air directly into the air separator provided above the roller mill whereas the coarse material portion falls downwardly out of the mill, and by means of a mechanical raising means is returned to the material entry of the roller mill, previously separating a selectable portion of the coarse material.
In all these known arrangements it has been constantly found in practice that it is very difficult to hold the wear due to the abrasive material within acceptable limits; moreover, for the pneumatic raising of at least the fine material portion from the roller mill into the air separator a considerable amount of energy is always required.